Method of inhibiting stress cracking

ABSTRACT

A method of inhibiting stress cracking in stainless steel articles exposed to a chloride-ion containing fluid environment in which the surface of the stainless steel article is contacted with a trace amount of an inorganic metal salt or with the metal corresponding to the cation of the salt so as to enlarge the anodic areas on the surface and increase the uniformity of the electrical potential of the surface thereby eliminating concentrated non-uniform attack on the surface and attendant cracking.

United States Patent Hess et al.

METHOD OF lNl-llBlTlNG STRESS CRACKING [76] Inventors: Daniel N. Hess, PO. Box 413,

Norris, Tenn. 37827; Ramon A. Bannister, 1710 Atlantic Ave., Sullivan Island. SC. 29482 [22] Filed: May 18, 1972 [21] Appl. N0.: 254,452

[52] US. Cl 2l/2.7 R; 176/38; 176/92 R; 252/387 [51] Int. Cl. C23f 9/02; C23f 11/00; C23f 11/08 [58] Field of Search 21/2.7 R; 176/38, 46, 54, 176/92 R [561 References Cited UNITED STATES PATENTS 2,837,474 6/1958 Cartledge 176/38 2,945,794 7/1960 Winters et al. 176/46 OTHER PUBLlCATlONS Berry, Warren E., Corrosion in Nuclear Applications, John Wiley and Sons, Inc., New York, N.Y., pages 318-321, P.O.S.L. TK9350.B46, 1971.

Primary Examiner-Morris 0.. Wolk Assistant ExaminerDale Lowercheck [57] ABSTRACT A method of inhibiting stress cracking in stainless steel articles exposed to a chloride-ion containing fluid environment in which the surface of the stainless steel article is contacted with a trace amount of an inorganic metal salt or with the metal corresponding to the cation of the salt so as to enlarge the anodic areas on the surface and increase the uniformity of the electrical potential of the surface thereby eliminating con centrated non-uniform attack on the surface and attendant cracking.

5 Claims, No Drawings METHOD OF HNHIBITHNG STRESS CRACKING The use of stainless steel in systems, particularly austenitic chromium-nickel stainless steels, starting with a modest use of a few decades ago. has proliferated into widespread use in many applications such as boilers, power plants and the like. For instance, stainless steel is widely used in such areas the petrochemical field, desalinization installations, and electric utility plants. From its inception, the use of stainless steel has always presented some problems in its industrial applications but, in particular, there is the pesky unpredictable problem of what is referred to as stress corrosion cracking," that is the occurrence of brittle fractures in steel.

While the term stress corrosion cracking is widely used to describe such stainless steel fractures, it is thought that the inclusion of the word corrosion in such a term is misleading. While undoubtedly cracking in stainless steel is a corroding process, corrosion of the type that causes cracking is focused on those specific areas of the stainless steel surface at which cracking occurs rather than throughout the stainless steel surface as the term corrosion implies. Therefore, it is thought that a more appropriate term for such cracking is stress cracking" and this term is used hereinafter to describe the occurrence of such fractures in steel.

The widespread use of stainless steel has focused extensive research and development efforts onto the problem of eliminating or substantially reducing such cracking since not only is stainless steel expensive but the cost of plant operation is magnified considerably when such plant must be shut down for replacement and repair of cracked stainless steel components. Furthermore. the incidence of industrial failures of austen itic stainless steel by stress cracking in chloride-bearing electrolytes has now reached a high degree of concern further adding to the urgent need for a solution to the problem.

It is commonly recognized that stress cracking in stainless steel results from the simultaneous action of tensile stress and a chloride-oxygen containing enviroment. The chloride-bearing electrolytes are recognized as being a source of cracking of austenitic stainless steels and although there is not complete agreement on the mechanism by which the chloride-ion attacks the steel, it is recognized as being the culprit.

The potency of the chloride-ion in producing stress corrosion cracking" has been well demonstrated by authorities working in the field. Such authorities are in general agreement that the cracking can occur readily in stainless steel specimens immersed in water containing only a few parts per milliom (ppm) of chloride-ions. The temperature of the chloride'ion containing water is not particularly significant in producing such cracking as, in addition to producing cracking at boiling temperatures and above, water temperatures as low as 75C. even in such dilute solutions have been known to produce cracking in stainless steel. Furthermore, such cracking appears to occur in specimens both stressed and unstressed and there appears to be little difference between the resistance of stainless steel to cracking between the various types of stainless steel.

Therefore, efforts have been made to eliminate the chloride-ions from the water or other solutions with which the stainless steel is contacted to thereby reduce stress cracking. In the nuclear reactor field, particularly in the power generating nuclear reactor plant, conduits 2 such as pipes, tubing, pipe fitting, etc. are generally formed from stainless steels and in such reactors, wherein both a primary and a secondary water system is utilized, it is extremely important to eliminate any cracking in the conduits of the secondary system since repair or replacement is difficult or at best extremely costly from the standpoint of down-time if stress cracking occurs. It is therefore the common practice to subject the water used in such secondary systems to extensive purification processes to eliminate all traces of the chloride-ion to avoid such cracking. It can be understood that due to the vast quantities of Water used in such systems such as a nuclear reactor secondary system, such water purification processes are extremely expensive and utilize equipment of extremely high cost. Furthermore, even though the processed water is subjected to such purification processes, some traces of the chloride-ion generally remain so that stress cracking cannot be precluded under present day practices.

As has been referred to above, the mechanism of stress cracking is not completely understood but it is generally believed that it is electrochemical in nature. The surface of the stainless steel is considered to consist oflarge cathodic areas and other very miniscule anodic areas. Thus, in an environment such as water containing a quantity of chloride-ions, an electric current flows through the anodic areas to the surrounding cathodic areas with the circuit being completed through the metal itself. As a result of the extreme narrowness of the anodic areas at the grain boundaries, the current density is relatively high in these areas so that the metal goes into solution rapidly at the anodic areas producing a site of structural weakness with resultant cracking.

Accordingly, a primary object of this invention is to provide a new and novel method of inhibiting stress cracking in stainless steel.

Another object of this invention is to provide a new and novel method of reducing stress cracking of stainless steels in contact with any solution containing chloride-ions.

Still another object of this invention is to provide a new and novel method of inhibiting stress cracking 01 stainless steels which requires the use of only minute quantities of a relatively inexpensive compound.

A further object of this invention is to provide a new and novel process for eliminating stress cracking in stainless steel constructions exposed to water containing chloride-ions such as in the secondary system of 2 pressurized water nuclear reactor, or industries utilizing stainless steel or the like.

This invention further contemplates the provision o a new and novel method of eliminating stress cracking in stainless steels which may be carried out in a simple and easy manner, which is extremely inexpensive botl in operation and materials used and which may be car ried out effectively through a wide variety of chloride containing electrolytes to prevent cracking in stainles: steel components so as to virtually eliminate any neet for maintanence or replacement of such component: over prolonged periods of time.

A still further object of this invention is to provide 2 new and novel method of eliminating stress cracking i1 stainless steel components in the secondary system 0 a pressurized water nuclear reactor which also may b1 used as an alarm system for signalling the presence 0 a leak between the primary and secondary systems 0 such a reactor.

Other objects and advantages of the invention will become apparent from the following description.

The objects stated above and other related objects of the invention are accomplished by the provision of a stainless steel article, the surface of which is exposed to a chloride-ion containing environment such as water containing chloride-ions. The surface of the stainless steel is contacted with a metal salt or the corresponding metal of the salt, the metal being selected from a group having a position in the electromotive force series such that the metal exerts an electrochemical potential on the exposed stainless steel surface so as to increase the uniformity of the electric potential of the stainless steel surface thereby eliminating concentrated non-uniform electrochemical attack on the isolated specific anodic areas of the surface and attendant cracking.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation may be best understood by reference to the following description.

In the practice of the invention, it is within the scope of the invention to introduce a compound such as a metal salt, preferably a nitrate such as mercuric nitrate monohydrate [Hg(NO H O] into a chloridecontaining electrolyte containing at least traces of the chloride-ion. The amount of mercuric nitrate added to the solution is preferably within the range of 2-l,000 parts per million (ppm) of fluid. The test results to follow indicate also that adjustment of the pH of the fluid environment by means of nitric acid (HNO has a beneficial effect in that the amount of mercuric nitrate required can be substantially reduced.

With this selected quantity of mercuric nitrate added to the electrolyte solution, cracking of the stainless steel immersed in the solution was virtually eliminated. It will be noted that in the test results to follow it was found that the temperature of the solution is relatively inconsequential as to the beneficial results obtained by the method of the invention.

It is thought that the introduction of the compound mercuric nitrate into the solution brings about an electro-chemical effect on the surface of the stainless steel with which the electrolyte solution is in contact. This electrochemical effect manifests itself in the enlargement of the rather limited number of small, specific anodic areas on the stainless steel surface where cracking characteristically occurs due to the high current density at these anodic areas. The mercuric nitrate or inhibitor appears to enlarge these anodic areas throughout the surface area of the stainless steel consequently reducing the cathodic areas on the surface with the result that the potential of the stainless steel surface is more uniform. Thus, current density at the grain boundaries or anodic areas is reduced by changing the electrical characteristics of the steel surface from one which is primarily cathodic to one which is uniformly, slightly more anodic thereby eliminating any high density current attack on highly anodic areas and attendant cracking. As a result of this uniform surface potential. localized attack does not occur. In addition, the steel surface is so passivated that there is no detectable increase in the rate of corrosion of the surface. Instead, it appears that the corrosion rate is decreased.

While the electrochemical theory advanced above appears to be well substantiated in the test results, a

concomitant or alternate phenomena may also result from the practice of the novel method of the invention. There is some evidence to indicate and this evidence appears in the test results of Table IV that a molecular coating of metal such as mercury, when mercuric nitrate is the additive, is formed on the stainless steel thereby serving to protect against attack by the chloride-ions in the fluid enviroment. It is thought that such a metal or mercury coating is only a few atoms in thickness, the metal being deposited out of the solution in minute quantitites on the stainless steel surface to produce the desired effect.

Various examples were conducted to show the beneficial results obtained with the method of the invention. In the examples set forth in Table l below, 2,000 milliliters of an aqueous solution containing 0.04 moles of UO SO 0.02 moles of H 50, and 0.005 moles of CuSC were provided at the temperature of approximately 100C. Chloride-ions were introduced in the solution as KCI or NaCl to provide a concentration of approximately 50 ppm of such ions.

This uranyl sulfate test solution was selected as being a useful solution by experimenters in the past for accelerated evaluation of the cracking behavior of stainless steel in a chloride-ion fluid enviroment. As the uranyl sulfate solution containing 25 ppm or greater of chloride-ions is very aggressive, it produces stainless steel cracking in a reasonable period of time, i.e., in about 500 hours whereas water alone containing chlorideions requires relatively longer periods of time to produce stainless steel cracking.

The solution was constantly stirred by a watersaturated air stream injected into the solution and a condenser was used to return steam as water back to the solution container. Make up water was added to maintain to original volume of the solution throughout the test.

The stainless steel specimens of Examples l-l0 were Type 347 stainless steel, hot-rolled, annealed and pickled with a 2B finish and with edges polished with -120 grit. The specimens were cut from a 1/l6 inch sheet and were U-shaped in configuration, the free ends being connected by a bolt. The specimens of Examples l l, 12 were of atype stainless steel identified as PH l5-7Mo and were /a inch thick with a 2D finish polished with 80-120 grit.

It will be noted in Example 1 l of Table I that the test conducted on this specimen was in three parts, identifiedas 11(a) (c). The specimen of Example ll remained in the solution for the time periods shown, cracks developing after 332 hours in 1 1(a); additional cracks occurring after 500 hours ll(b); and finally in l 1(c) following the introduction of the amount of additive shown into the solution no additional cracks on the specimen occurred after 1,000 hours. The specimens of Examples l3, 14 were of the same type of stainless steel as in Examples 1 l, 12 except that the specimens were polished after l750F. heat treatment, formed into a U-shaped configuration and pickled after 950F. heat treatment. The specimens of Examples 15-17 were Type AM 350 stainless steel. l/l6 inch thick. heat treated for 2 hours at l950F., pickled and polished with 80-120 grit, the specimen of Example l7 being in the solution-cooled and tempered condition. In Example l2(a) no cracking occurred within 500 hours when the specimen was placed in the test solution containing the additive shown. However, when the same specimen was placed in the test solution containing no additive cracking occurred within the next 500 hours as shown in Example l2(b).

The results of these examples are set forth in Table Type 304 stainless steel, as received, 1/64 inch thick and sheared. hot-rolled, annealed and pickled. The steel specimens of Examples 7-23 were Type 347 stainless steel, l/l6 inch thick with a 2B finish. These specilbelow: 5 mens were ln the same as received condition of the TABLE I Experiment Type of Additive Cracking Time Corrosion No. Stainless (ppm) (hrs) Rate Steel (mm 1 347 none yes 50 2 347 none yes 384 19 3 347 70 Hg (NO;,) yes 193 I8 4 347 150 HgCL, yes 247 13 5 347 500 HgSO, no 360 3 6 347 500Hg(Cl). i yes 96 80 7 347 500 Hg(Cl) yes 96 56 8 347 500 Hg(NO;,) no 2000 0.02 9 347 500 Hg(N0 no 2000 0.01 10 347 500 Hg(NO no 2000 0.02 ll(a) PH l5-7Mo none yes 332 (b) PH l5-7Mo none yes 500 (c) PH l5-7Mo 500 Hg(NO no 1000 l2(a) PH l5-7Mo 500 Hg(NO,-,) no 500 l2(b) PH l5-7Mo none yes 500 13 PH l5-7Mo none yes 69 14 PH l5-7Mo 500 Hg(NO,,)-; no 2000 0.29 15 AM 350 none no 1000 0.1 I I6 AM 350 500 Hg(NO;,) no 1000 0.03 17 AM 350 500 Hg(NO,,) no 1000 0.04

ous solution of water into which chloride-ions as KCl or NaCl in the quantity of 50 ppm was introduced. The steel specimens used in the examples set forth in Table ll were Type 347 stainless steel cut from 1/16 inch sheet hot-rolled, annealed and pickled with a 2B finish.

specimens of Table l i.e., hot-rolled, annealed and pickled, edges polished with 80-120 grit. in Example 20, the specimens of Example 19 were placed in a solution that contained no Additive No. l, the inhibitor.

The results of these examples are set forth in Table 11] below:

The edges were polished with a 80-420 grit. Mercuric TABLE l was addfd to the water Example Additive No. l Additive No. 2 Time Cracked solutlon at 300 C. and under 150 p51 of oxygen pres- N0. H (NO=.)2.H-iO ppm(H ii) sure in the amounts shown to obtain the novel results (ppm 115 g) of the invention. These examples are set forth as fol- I 0 0 yes lows in Table ll below: 40 2 0 0 a yes TABLE ll Experiment Type of Additive Cracking Time Corrosion No. Stainless (ppm) (hrs) Rate Steel 1 347 300 Hg(NO,-,) no 2376 0 2 347 150 l-l (N0,,) no 1000 0 It will be noted from the examples of Table II that the i 8 8 3 addition of mercuric nitrate to the solution completely 5 300 0 5 no eliminated cracking of the stainless steel specimens al- 6 300 0 122 no though the test was conducted for extremely long peri- 8 6 ods of time. Also, the specimens showed no measurable 9 0 0 6 yes corrosion further exhibiting the corrosion preventing 8 results of the novel method of the invention. 13 100 o 45 in One of the classic tests used to evaluate the cracking 8 g; susceptibility of stainless steel is the immersion of a 15 100 0:003 t, steel sample in boiling 42 percent MgCl solution which ,0 16 200 0 l q no is a very aggressive solution. Consequently. in order to 8 :8 show the efficacy of the method of the invention, a se- 19 20 0.001 192 I10 ries of examples were conducted on stainless steel spec- 3 8 008 ,33 imens immersed in such boiling magnesium chloride 33 30 0:15 54 its solution both with and without the addition of mercuric 65 33 30 nitrate, one novel compound used in the method of the invention. The solution used was 92 milliliters of water in which 812 grams of magnesium chloride was dissolved. The steel specimens of Examples l-(l were It will be noted in the Examples of Table III that without the addition of mercuric nitrate to the magnesium chloride fluid enviroment, cracking of the stainless steel specimens occurred quickly, within as little as 2 hours, such as in Example 7. The addition 'of the quantities of mercuric nitrate shown brought about vast improvement in the time before cracking and in most examples, no cracking whatsoever of the steel specimens resulted during the period of the test. As indicated above, nitric acid (Additive No. 2) was introduced in the fluid enviroment to adjust the pH of the solution.

In order to corroborate what is believed to be one of the phenomena of the invention, that is the protective action of mercury on the stainless steel surface when the additive used is mercuric nitrate, a series of examples were conducted using mercury metal. In the examples of Table IV below, Type 347 stainless steel specimens, prepared similarly to those specimens of Table II, were floated on a puddle of mercury metal that had been poured into the 42 percent magnesium chloride solution. As set forth in the Examples of Table IV, although cracking occurred, the period of time required for each specimen to crack was considerable and the periods were substantially longer than the cracking times of those specimens of Table lll where no additive was used as in Examples 1-3 and 7-9. Table IV is as follows:

While there has been described what at present is believed to be the preferred embodiment of the invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the invention.

Having thus described the invention, what is claimed l. A method of inhibiting stress cracking of stainless steel in a chloride-ion fluid environment comprising, contacting a stainless steel surface in a chloride-ion containing fluid environment with mercuric nitrate in an effective amount to inhibit stress cracking of said stainless steel surface.

2. A method in accordance with claim 1 comprising the further step of adjusting the pH of said chloride ioncontaining fluid by the addition thereto of from 0.00] to 0.008 parts per million of said chloride ioncontaining fluid of nitric acid.

3. A method in accordance with claim 1 wherein said contacting step is carried out by adding said mercuric nitrate to said chloride-ion containing fluid environment.

4. A method in accordance with claim 3 wherein said chloride-ion containing fluid enviroment comprises an aqueous solution containing a quantity of chlorideions.

5. A method in accordance with claim 4 wherein the quantity of mercuric nitrate added is within the range o'f-'2l00O parts per million of said chloride-ion containing fluid enviroment. 

1. A METHOD OF INHIBITING STRESS CRACKING OF STAINLESS STEEL IN A CHLORIDE-ION FLUID ENVIRONMENT COMPRISING, CONTACTING A STAINLESS STEEL SURFACE IN A CHLORIDE-ION CONTAINING FLUID ENVIRONMENT WITH MERCURIC NITRATE IN AN EFFECTIVE AMOUNT OF INHIBIT STESS CRACKING OF SAID STAINLESS STEEL SURFACE.
 2. A method in accordance with claim 1 comprising the further step of adjusting the pH of said chloride ion-containing fluid by the addition thereto of from 0.001 to 0.008 parts per million of said chloride ion-containing fluid of nitric acid.
 3. A method in accordance with claim 1 wherein said contacting step is carried out by adding said mercuric nitrate to said chloride-ion containing fluid environment.
 4. A method in accordance with claim 3 wherein said chloride-ion containing fluid enviroment comprises an aqueous solution containing a quantity of chloride-ions.
 5. A method in accordance with claim 4 wherein the quantity of mercuric Nitrate added is within the range of 2-1000 parts per million of said chloride-ion containing fluid enviroment. 